Extended AP_MotorsTri class to take in pointer to rc_tail servo (rc_7) and we use this servo's REV parameter to determine whether to reverse the output to the tail servo or not
1. it's step towards rearchitecting the current code base ahead of a move to RTOS.
2. internally it uses a MatrixTable for the Quad, Octa, OctaQuad, Y6 frames.
3. it implements the missing stability patch for Octa and OctaQuads (still missing for Y6)
Later check-ins will incorporate into the main ArduCopter code.